Accurate flow measurement is increasingly important in many fields. For example, in the field of civil aircraft, increases in accuracy in the flow measuring systems used to meter fuel mean that less fuel has to be carried by aircraft to compensate for measuring inaccuracies, which in turn leads to improvements in fuel consumption.
Most mechanical fluid flow measuring devices intrude to some extent into the flow they are set up to measure. This can undesirably affect the flow, expose the measuring device to possibly harmful fluids, and complicate installation of measuring devices into existing fluid systems.
In various industries low resolution NMR machines are used to analyse fluids. For example, the food processing industry uses benchtop NMR systems to detect foreign bodies in production lines. These systems are sometimes enhanced by the addition of special coils which generate one or more field gradients. This technique allows flow cross sections to be imaged. See e.g. Van Dijk, P., (1984), J. Comput. Assist. Tomogr., Vol. 8 (3), 429.
Another known NMR technique for analysing fluid flows (see e.g. Singer, J. R., (1972), Science, Vol. 175, 794) uses a time of flight approach to determine mass flow rates.
Compared with mechanical fluid flow measuring devices, both these NMR techniques have the advantage of no moving parts. However, the gradient coil requires additional (generally heavy and expensive) field gradient amplifiers and control circuits, and the time of flight approach requires a separate detector coil and is limited by the relaxation time of the fluid.
Thus, it would be desirable to have an NMR technique for analysing fluid flows which is accurate, non-intrusive and is capable of providing flow rate information.
It is useful at this point to provide a brief overview of the relevant NMR theory.
Certain atomic nuclei possess angular momentum and the quantum property of “spin”. Because the nuclei also carry a charge, specifically a positive charge, there is a magnetic moment associated with this spin. When placed in a magnetic field, these nuclei, which might be referred to as the nuclear “magnets”, tend to align with the field direction. Only certain orientations are possible—two in the case of a spin ½ nucleus such as a proton.
The energy difference between the orientations of the nuclei (“Zeeman splitting”) depends linearly on the strength of the magnetic field B. Transitions between the two orientations can be induced when the frequency of an applied oscillating magnetic field (normally electromagnetic radiation such as a radio frequency (RF) signal), exactly matches the energy difference. This so called resonance condition, is defined by the Larmor equation:ω=γBwhere ω is the angular frequency of the oscillating magnetic field (electromagnetic radiation) and γ, referred to as the magnetogyric ratio, is a constant for a particular nuclear species.
Different nuclei have different values of γ and so resonate at different frequencies in a magnetic field of given strength. For example, at 11.7 T, resonant frequencies for the following nuclei are: 1H—500 MHz; 13C—125.7 MHz; 27Al—130.3 MHz; 29Si—99.3 MHz; 51V—131.5 MHz; 53Cr—28.3 MHz; 55Mn—123.3 MHz; 59Co—118.1 MHz, 95Mo—32.6 MHz; 107Ag—23.3 MHz and 183W—20.8 MHz.
The magnetic field B in the Larmor equation given above is the actual field strength at the nucleus and includes susceptibility effects arising from the bulk magnetic properties of the sample, local variations in these effects due to sample heterogeneity, and the screening effect of the electrons that surround the nucleus itself. Thus:B=B0(1+χ)where χ is the magnetic susceptibility and B0 is the applied magnetic field.
In traditional high resolution NMR, it is the contribution of the screening electrons to χ that gives the technique its power to analyse chemical structure: the same nucleus (e.g. 1H) will experience different magnetic fields depending on the chemical environment, so that chemically distinct nuclei resonate at slightly different frequencies. The range of these chemical shifts for any particular nucleus is, however, small: 0-10 ppm covers most 1H resonances of interest. In order to resolve them, the main applied magnetic field B0 must be maintained homogeneous over the sample volume. A few parts in 109 are commonplace and a few parts in 1010 achievable with spinning samples under ideal conditions.
Modern NMR machines mostly use so-called pulsed NMR spectroscopy. This involves creating a first, non-oscillating magnetic field of a predetermined field strength across the sample, and intermittently exposing the sample to a second, oscillating magnetic field orthogonal to the first to generate an NMR signal. The relatively short pulse width (typically of the order of μs) of the intermittent field makes it possible to simultaneously detect a range of frequencies in the NMR signal.
The trend in modern NMR spectroscopy is towards pulsed NMR with high non-oscillating field strengths and high resolutions. Largely this has been made possible by developments in the technology of superconducting magnets.
In contrast, EP 1191330 describes an approach for detecting anomalies in fluid systems that is suitable for the performance of pulsed NMR at low non-oscillating field strengths (e.g. 1.5 T or less).
In particular, EP 1191330 proposes that such anomalies can be detected and analysed in two ways. The first way is termed “indirect detection” and involves analysing the influence the anomalies have on a signal from the fluid rather than analysing a signal from the anomalies themselves. This is particularly useful for the detection of inhomogeneities, such as particulates, in the fluid. The approach is possible where the particulates have a different magnetic susceptibility than the fluid, because they will then cause local non-uniformities in the magnetic field. This in turn modifies the NMR signal from the fluid, manifesting itself, for instance, in changes of line-width and/or position of the fluid resonance seen in the NMR frequency domain.
The second way is termed “direct detection” and, to the extent that the NMR signal that is detected and analysed does derive directly from a contaminant or additive in the fluid system, it is closer to traditional NMR techniques. Direct detection can be used to detect inhomogeneities or dissolved species in the fluid.